Preparation of organic acid salts of cationic surfactants

ABSTRACT

The invention pertains to a highly efficient process for preparing a very pure food-grade organic acid salt of a cationic surfactant derived from the condensation of a fatty acid with an esterified dibasic amino acid. The process involves the following steps:
         (a) providing a reaction mixture comprising a mineral acid salt of the cationic surfactant and an alkali and/or alkaline earth metal salt of the food-grade organic acid;   (b) allowing the reaction between the mineral acid salt of the cationic surfactant and the alkali and/or alkaline earth metal salt of the food-grade organic acid salt to proceed until substantially all of the mineral acid salt of the cationic surfactant has been converted to the food-grade organic acid salt of the cationic surfactant; and   (c) recovering the food-grade organic acid salt of the cationic surfactant from the reaction mixture.

FIELD OF THE INVENTION

The present invention relates to a highly efficient method forconverting a mineral acid salt of a cationic surfactant into afood-grade organic acid salt of the cationic surfactant. The resultantorganic acid salt is very useful for incorporation into a wide varietyof food products to thereby inhibit the proliferation of different typesof microorganisms such as bacteria, fungi and yeasts to thereby increasethe useful life of such food products, particularly perishable foodproducts.

BACKGROUND OF THE INVENTION

Mineral acid salts of cationic surfactants derived form the condensationof fatty acids and esterified dibasic amino acids and their use for thepreservation of foodstuffs are known in the prior art—see, e.g., U.S.Pat. Nos. 5,780,658, 7,087,769 and 7,407,679. A useful cationicsurfactant is the ethyl ester of lauramide-arginine (hereinafterreferred to as the ethyl ester of N^(α)-lauroyl-L-arginine); such esteris also referred to herein by the term “LAE”. The anions of such mineralacid salts are typically Br⁻, Cl⁻ or HSO₄ ⁻, with the most typical beingCl⁻.

It is also known in the prior art to prepare food-grade organic acidsalts rather than the mineral acid salts of the cationic surfactants. WO2007/014580 discloses that such organic acids may be, e.g., citric acid,lactic acid, acetic acid, fumaric acid, maleic acid, gluconic acid,propionic acid, benzoic acid, carbonic acid, glutamic acid or otheramino acids, lauric acid and fatty acids such as oleic acid and linoleicacid.

It has been found that the food-grade organic acid salts of the cationicsurfactants, rather than the mineral acid salts of the cationicsurfactants, are especially useful for the preservation of foodstuffs.However, the process for preparing the food-grade organic acid saltsdisclosed in WO 2007/014580 leaves a lot to be desired. The yields arepoor and moreover, the product is impure. For example, in the case ofLAE being converted from the monohydrochloride salt to the acetate salt,the product is contaminated with a significant amount of the byproductN^(α)-lauroyl-arginine (“LAS”) acetate.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a highly efficient processfor the preparation of food-grade organic acid salts of the foregoingcationic surfactants.

It is a further object of the invention to provide a process resultingin the production of food-grade organic acid salts of the foregoingcationic surfactants in exceptionally high yields.

It is yet an additional object of the invention to provide a processresulting in the production of exceptionally pure food-grade organicacid salts of the foregoing cationic surfactants.

These and other objects have been met by this invention whose detailsare set forth below.

DETAILS OF THE INVENTION

The process of the invention pertains to the preparation of an organicacid salt of a cationic surfactant derived from the condensation of afatty acid with an esterified dibasic amino acid comprising the stepsof:

-   -   (a) providing a reaction mixture comprising a mineral acid salt        of the cationic surfactant and an alkali and/or alkaline earth        metal salt of the food-grade organic acid;    -   (b) allowing the reaction between the mineral acid salt of the        cationic surfactant and the alkali and/or alkaline earth metal        salt of the food-grade organic acid salt to proceed until        substantially all of the mineral acid salt of the cationic        surfactant has been converted to the food-grade organic acid        salt of the cationic surfactant; and    -   (c) recovering the food-grade organic acid salt of the cationic        surfactant from the reaction mixture.

The process of the invention may be carried out in two modes: In a firstmode, the reaction between the mineral acid salt of the cationicsurfactant and the alkali and/or alkaline earth metal salt of thefood-grade organic acid takes place “neat”, i.e., the mineral acid saltof the cationic surfactant is present in the reaction mixture in amolten state and a diluent is not present in the reaction mixture. Inthe first mode, the mineral acid salt of the cationic surfactant isheated, e.g., to a temperature of about 60-75° C., in order to melt itand the alkali and/or alkaline earth metal salt of the food-grade acidis added to the molten mass, with agitation.

The reaction between the mineral acid salt of the cationic surfactantand the alkali and/or alkaline earth metal salt of the food-grade acidis exothermic. When the temperature of the reaction mixture remainsstable, the reaction is complete, i.e., substantially all (i.e., atleast about 95%) of the mineral acid salt of the cationic surfactant hasbeen converted into the food-grade organic acid salt of the cationicsurfactant. Another indication that the reaction is complete is that thevolume of solids in the reaction flask decreases since the volume of theby-product of the reaction, i.e., the alkali and/or alkaline earth metalhalide salt, is less than the volume of the desired product, i.e., thefood-grade organic acid salt of the cationic surfactant. Typically, areaction temperature of about 20 to about 100° C., preferably 40-80° C.,and a residence time of about 0.5 to about 10 hours, preferably 1-6hours, are required for completion of the reaction.

In order to insure that the reaction is complete after step (b) of theprocess has been carried out, a diluent of the type described below ispreferably added to the reaction mass before commencing step (c) of theprocess and agitation is continued for a period of about 0.5 to about 4hours at a temperature of 40-80° C.

In the second mode of carrying out the process of the invention, thereaction mixture in step (a) will initially comprise the mineral acidsalt of the cationic surfactant and a diluent of the type describedbelow. The initial reaction mixture is stirred for several minutes andheated to a temperature of about 40 to about 80° C. Thereafter, thealkali metal and/or alkaline earth metal salt of the food-grade organicacid is slowly added with agitation to the reaction mixture. Here again,an exothermic reaction takes place, but the temperature rise is less(than when the reaction is conducted “neat”), due to the moderatingpresence of the diluent. When the temperature of the reaction mixtureremains stable, the reaction is complete and substantially all (i.e., atleast about 95%) of the mineral acid salt of the cationic surfactant hasbeen converted into the food-grade organic acid salt of the cationicsurfactant. Another indication that the reaction is complete is that thevolume of solids in the reaction flask decreases since the volume of theby-product of the reaction, i.e., the alkali and/or alkaline earth metalhalide salt, is less than the volume of the desired product, i.e., thefood-grade organic acid salt of the cationic surfactant. Typically, areaction temperature of about 20 to about 100° C., preferably 40-80° C.,and a residence time of about 0.5 to about 10 hours, preferably 1-6hours, are required for completion of the reaction for completion of theprocess in the second mode.

Step (c) of the process of the invention is readily accomplished byfiltering the reaction mixture while it is hot. To insure that nofood-grade organic acid salt of the cationic surfactant is entrained inthe by-product precipitate (i.e., the alkali and/or alkaline earth metalhalide salt), the precipitate may be washed one or more times withadditional quantities of the diluent. The diluent is then distilled offfrom the filtrate (plus any washings) under vacuuo, e.g., at 40-60° C.and 40-60 mm Hg.

Regardless of which mode of the process of the invention is practiced,the yield of the product will typically be in the range of 95-99% oftheory and the purity of the product will be typically be in the rangeof 98-99%.

In general, the diluent is utilized in the reaction mixture in a ratioof about 2 to about 10 w/v, preferably 3 to 6 w/v, based on the weightof the mineral acid salt of the cationic surfactant. Useful diluentsinclude C₁-C₄ straight chain alcohols, C₁-C₄ branched chain alcohols,C₁-C₄ straight chain esters of C₁-C₈ monobasic organic acids, branchedchain esters of C₁-C₈ monobasic organic acids, and mixtures thereof.Preferably, the diluent is a straight-chain lower alcohol such asethanol, isopropanol, n-propanol, n-butanol and mixtures thereof.Alternatively, the diluent is an ester such as ethyl acetate, methylacetate, n-butyl acetate, ethyl formate and mixtures thereof. Theparticularly preferred diluent to be employed in step (a) is ethylacetate. It is particularly preferred that the reaction mixture notcontain any water.

The mineral acid salt of the cationic surfactant is preferably themonohydrochloride salt of the selected cationic surfactant. The cationicsurfactant may be any of those known in the prior art, such as anN^(α)-(C₁-C₂₂) alkanoyl di-basic amino acid (C₁-C₂₂) alkyl ester. Thepreferred cationic surfactants are N^(α)-lauroyl-L-arginine ethyl ester,N^(α)-lauroyl-L-histidine ethyl ester and N^(α)-lauroyl-L-tryptophanethyl ester. The particularly preferred mineral acid salt of thecationic surfactant is N^(α)-lauroyl-L-arginine ethyl estermonohydrochloride, i.e., LAE hydrochloride.

Typically, the alkali and/or alkaline earth metal salt of the selectedfood-grade organic acid will be employed in a molar amount of about 1 toabout 1.5 moles, preferably 1 to 1.3 moles, per mole of the mineral acidsalt of the selected cationic surfactant. The alkali metal may be, e.g.,sodium, potassium, lithium, etc., while the alkaline earth metal maycalcium, magnesium, etc. Preferably, the salt of the food-grade organicacid is the sodium salt.

The food-grade organic acid may be a carboxylic acid such as acetic,benzoic, butyric, capric, caproic, caprylic, citric, formic, fumaric,gluconic, glyceric, glycolic, heptanoic, lactic, lauric, linoleic,maleic, malic, myristic, nonanoic, oleic, palmitic, propionic,salicyclic, sorbic, stearic, tartaric, undacanoic, undecylenic orvaleric. Preferred food-grade organic acids are acetic, citric, lacticand lauric. The particularly preferred food-grade organic acid is aceticacid.

The following non-limiting examples shall serve to illustrate thevarious embodiments of this invention. Unless otherwise indicated, allamounts and percentages are on a weight basis.

Example 1

A 1-liter flask was set up with a stirrer, thermometer, condenser andvacuum distillation unit. Added to the flask with stirring were added 42g (0.1 mole) LAE hydrochloride and 350 g of distilled water. Thereafter,9.5 g (0.115 mole) of sodium acetate were added and the reaction mixturewas stirred for six hours at a temperature of 20-25° C. It was notedthat some white precipitate was present. Thereafter, 300 g of ethylacetate were added and the mixture was stirred for two hours at atemperature of 20-25° C.

The stirring was stopped and the mixture was poured into a separatoryfunnel and the top product phase was removed (it was noted that a smallamount of white precipitate was present in the product phase). The ethylacetate solvent was distilled off at a temperature of 75-80° C. under50-60 mm Hg vacuum. The results were as follows:

Product weight 32 g % sodium chloride  2.1% % LAS  1.9% % LAEhydrochloride 14.1% % LAE acetate 82.0% % Yield 60.0%

It was concluded that the use of water in the reaction mixture wasdetrimental since the yield of the LAE acetate was only 60% and theproduct was significantly contaminated with by-products and unreactedLAE hydrochloride.

Example 2

Example 1 was repeated, except that after the addition of the sodiumacetate, the reaction mixture was heated to 75-80° for six hours (whiteprecipitate was again present in the reaction mixture). The results wereas follows:

Product weight 35 g % sodium chloride  2.4% % LAS 21.0% % LAEhydrochloride 11.0% % LAE acetate 55.4% % Yield 44.6%

It was concluded that the use of water in the reaction mixture coupledwith the elevated reaction temperature results in very poor productyield and further that the product had a very high level ofcontamination from by-products and unreacted LAE hydrochloride.

Example 3

This example was carried out using the same equipment set-up as isdescribed in Example 1. 42 g (0.1 mole) of LAE hydrochloride and 300 gof ethyl acetate were placed in the reaction flask and stirred.Thereafter, 9.5 g (0.115 mole) of sodium acetate were added withstirring. The reaction mixture was then heated, while stirring, to 50°C. and maintained at such temperature for six hours. The reactionmixture was then allowed to come to room temperature and filtered toremove the insoluble precipitate. The insoluble precipitate was thenwashed with 50 g of ethyl acetate to remove any product that may havebeen entrained in the precipitate. The ethyl acetate filtrate fractionswere then combined and the ethyl acetate was distilled off at 75-80° C.and 40 to 50 mm Hg vacuum.

The results were as follows:

Product weight 38 g % sodium chloride  0.4% % LAE hydrochloride  3.1% %LAE acetate 95.5% % Yield 82.0%

The product had a melting point of 77-79° C.

The results of Example 3 demonstrate that the process of the inventionusing ethyl acetate as the diluent, a reaction temperature of 50° C. anda reaction time of six hours, is highly efficient and product with highpurity and in high yield can be obtained.

Example 4

Example 3 was repeated using a reaction temperature of 75-80° C. and areaction time of four hours. The results were as follows:

Product weight 43.1 g % LAE hydrochloride  0.7% % LAE acetate 98.2% %Yield 95.5%The product had a melting point of 80-82° C.

The results of Example 4 demonstrate that the process of the inventionusing ethyl acetate as the diluent, a reaction temperature of 75-80° C.and a reaction time of four hours can produce product with excellentpurity and excellent yield.

Example 5

Example 4 was repeated except that the reaction time was six hoursrather than four hours. The results were as follows:

Product weight 44.1 g % LAE hydrochloride 0.32% % LAE acetate 99.2% %Yield 98.5%The product had a melting point of 81-83° C.

The results of Example 5 demonstrate that the process of the inventionusing ethyl acetate as the diluent, a reaction temperature 75-80° C. anda reaction time of six hours can produce product with exceptional purityand exceptional yield.

Example 6

This example was carried out using the same equipment set-up as isdescribed in Example 1. 42 g (0.1 mole) of LAE hydrochloride were addedto the flask and heated to 65-70° C. to melt it. Thereafter, 23.34 g(0.105 mole) of powdered sodium laurate were added to the flask withagitation (the reaction was exothermic—the temperature rose to 85° C.).The reaction mixture was maintained at 80-85° C. for 30 minutes tocomplete the reaction. Subsequently, 300 g of ethyl acetate were addedto the flask at 80° C. and the contents of the flask were agitated andheld at 75° C. for 30 minutes. The contents of the flask were thenfiltered hot to remove the by-product salt precipitate. The by-productprecipitate was washed with additional ethyl acetate diluent to removeany entrained product. The ethyl acetate diluent fractions were thenremoved under vacuuo at 70-75° C. and 50-60 mm Hg. The yield of theproduct was 57.31 g (96.5% yield). The HPLC assay of the product was asfollows:

% LAE Laurate 98.5%  % LAS 0.8% % Lauric Acid 0.4%

Example 7

Example 6 was repeated using 42 g of LAE hydrochloride which was heatedto 65-70° C. to melt it. Thereafter, 22.5 g (0.105 mole) of powderedmonosodium citrate were added with agitation (the reaction wasexothermic—the temperature rose to 82° C.). The reaction mixture wasmaintained at 80-85° C. for 30 minutes to complete the reaction.Subsequently, 300 g of ethyl acetate were added to the flask at 80° C.and the contents of the flask were agitated and held at 75° C. for 30minutes. The contents of the flask were then filtered hot to remove theby-product salt. The by-product precipitate was washed with additionalethyl acetate diluent to remove any entrained product. The ethyl acetatediluent fractions were then removed under vacuuo at 70-75° C. and 50-60mm Hg. The yield of the product was 57.1 g (97.8% yield). The HPLC assayof the product was as follows:

% LAE Citrate 98.5%  % LAS 0.6% % Citric Acid 0.2%

Example 8

Example 6 was repeated using 42 g of LAE hydrochloride which was heatedto 65-70° C. to melt it. Thereafter, 11.8 g (0.105 mole) of powderedsodium lactate were added with agitation (the reaction wasexothermic—the temperature rose to 83° C.). The reaction mixture wasmaintained at 75-85° C. for 30 minutes to complete the reaction.Subsequently, 300 g of ethyl acetate were added to the flask at 75° C.and the contents of the flask were agitated and held at 75° C. for 30minutes. The contents of the flask were then filtered hot to remove theby-product salt. The ethyl acetate diluent was then removed under vacuuoat 70-75° C. and 50 mm Hg. The yield of the product was 46.5 g (96.7%yield). The HPLC assay of the product was as follows:

% LAE Lactate 98.5% % LAS  0.6% % Lactic Acid 0.45%

The foregoing examples are provided for illustrative purposes only andshould not be construed as representing any limitations on the scope ofthe invention. The scope of the invention is defined by the claims whichfollow.

1. A process for the preparation of a food-grade organic acid salt of acationic surfactant derived from the condensation of a fatty acid withan esterified dibasic amino acid comprising the steps of: (a) providinga reaction mixture comprising a mineral acid salt of the cationicsurfactant and an alkali and/or alkaline earth metal salt of thefood-grade organic acid; (b) allowing the reaction between the mineralacid salt of the cationic surfactant and the alkali and/or alkalineearth metal salt of the food-grade organic acid salt to proceed untilsubstantially all of the mineral acid salt of the cationic surfactanthas been converted to the food-grade organic acid salt of the cationicsurfactant; and (c) recovering the food-grade organic acid salt of thecationic surfactant from the reaction mixture.
 2. The process of claim 1wherein the cationic surfactant comprises an N^(α)—(C₁-C₂₂) alkanoyldi-basic amino acid (C₁-C₂₂) alkyl ester.
 3. The process of claim 2wherein the cationic surfactant comprises an ester selected from thegroup consisting of N^(α)-lauroyl-L-arginine ethyl ester,N^(α)-lauroyl-L-histidine ethyl ester and N^(α)-lauroyl-L-tryptophanethyl ester.
 4. The process of claim 3 wherein the cationic surfactantcomprises N^(α)-lauroyl-L-arginine ethyl ester.
 5. The process of claim1 wherein the mineral acid salt of the cationic surfactant comprises thehydrochloride salt of the cationic surfactant.
 6. The process of claim 1wherein the reaction mixture further comprises a diluent.
 7. The processof claim 6 wherein the mineral acid salt of the cationic surfactant andthe diluent are present in the reaction mixture in a ratio of about 2 toabout 10 w/v, based on the weight of the mineral acid salt of thecationic surfactant.
 8. The process of claim 1 wherein step (b) iscarried out at a temperature in the range of about 20 to about 100° C.for a period of time of about 0.5 to about 10 hours.
 9. The process ofclaim 1 wherein step (a) is carried out in the absence of any diluentand the mineral acid salt of the cationic surfactant is present in thereaction mixture in a molten state.
 10. The process of claim 1 whereinthe alkali and/or alkaline earth metal salt of the food-grade organicacid is employed in a molar amount of about 1 to about 1.5 moles permole of the mineral acid salt of the cationic surfactant.
 11. Theprocess of claim 1 wherein the alkali metal comprises sodium.
 12. Theprocess of claim 1 wherein the diluent is selected from the groupconsisting of C₁-C₄ straight chain alcohols, C₁-C₄ branched chainalcohols, C₁-C₄ straight chain esters of C₁-C₈ monobasic organic acids,branched chain esters of C₁-C₈ monobasic organic acids, and mixturesthereof.
 13. The process of claim 12 wherein the diluent comprises analcohol selected from the group consisting of ethanol, n-propanol,isopropanol and n-butanol.
 14. The process of claim 12 wherein thediluent comprises an ester selected from the group consisting of ethylacetate, methyl acetate, n-butyl acetate and ethyl formate.
 15. Theprocess of claim 14 wherein the ester comprises ethyl acetate.
 16. Theprocess of claim 1 wherein the food-grade organic acid is selected fromthe group consisting of acetic acid, benzoic acid, butyric acid, capricacid, caproic acid, caprylic acid, citric acid, formic acid, fumaricacid, gluconic acid, glyceric acid, glycolic acid, heptanoic acid,lactic acid, lauric acid, linoleic acid, maleic acid, malic acid,myristic acid, nonanoic acid, oleic acid, palmitic acid, propionic acid,salicyclic acid, sorbic acid, stearic acid, tartaric acid, undacanoicacid, undecylenic acid and valeric acid.
 17. The process of claim 16wherein the food-grade organic acid is selected from the groupconsisting of acetic acid, citric acid, lactic acid and lauric acid. 18.The process of claim 17 wherein the food-grade organic acid comprisesacetic acid.
 19. A process for the preparation of a food-grade organicacid salt of a cationic surfactant derived from the condensation of afatty acid with an esterified dibasic amino acid comprising the stepsof: (a) providing a reaction mixture comprising a mineral acid salt ofthe cationic surfactant in a molten state and an alkali and/or alkalineearth metal salt of the food-grade organic acid; (b) agitating thereaction mixture of step (a) and allowing the reaction between themineral acid salt of the cationic surfactant and the alkali and/oralkaline earth metal salt of the food-grade organic acid salt to proceeduntil substantially all of the mineral acid salt of the cationicsurfactant has been converted to the food-grade organic acid salt of thecationic surfactant; (c) adding a diluent to the reaction mixtureresulting from step (b) and agitating the mixture of the diluent and thereaction mixture at a temperature in the range of about 40 to about 80°C. for a period of time of about 0.5 to about 10 hours; and (d)recovering the food-grade organic acid salt of the cationic surfactantfrom the reaction mixture resulting from step (c).
 20. A process for thepreparation of a food-grade organic acid salt of a cationic surfactantderived from the condensation of a fatty acid with an esterified dibasicamino acid comprising the steps of: (a) providing a reaction mixturecomprising a mineral acid salt of the cationic surfactant and a diluent;(b) adding an alkali and/or alkaline earth metal salt of the food-gradeorganic acid to the reaction mixture resulting from step (a); (c)agitating the reaction mixture of step (b) and allowing the reactionbetween the mineral acid salt of the cationic surfactant and the alkaliand/or alkaline earth metal salt of the food-grade organic acid salt toproceed until substantially all of the mineral acid salt of the cationicsurfactant has been converted to the food-grade organic acid salt of thecationic surfactant; and (d) recovering the food-grade organic acid saltof the cationic surfactant from the reaction mixture resulting from step(c).